1. Field of the Disclosure
The present disclosure relates to a surface emitting laser, and more particularly, to an end-pumped vertical external cavity surface emitting laser (VECSEL) in which a pump laser beam is incident on a laser chip at a right angle.
2. Description of the Related Art
Among semiconductor lasers, an edge emitting laser emits a laser beam parallel to a substrate, and a surface emitting laser or vertical cavity surface emitting laser (VCSEL) emits a laser beam perpendicular to a substrate. The VCSEL has a high coupling efficiency because it can oscillate at a single longitudinal mode of a narrow spectrum and also it has a small emission angle. Further, the VCSEL can be easily integrated into other devices due to its structure. However, it is more difficult for the VCSEL than for the edge emitting laser to oscillate at a single traverse mode. The VCSEL requires a narrow oscillation region for single traverse mode operation, causing a low emission power.
To retain the advantages of the VCSEL while adding high emission power, a vertical external cavity surface emitting laser (VECSEL) has been developed. The VECSEL has a gain region that is increased by replacing an upper mirror with an external mirror so that the VECSEL can obtain a high emission power of tens of watts or more.
FIG. 1 is a schematic sectional view of an optical pumping type VECSEL according to the related art. Referring to FIG. 1, an optical pumped VECSEL 10 includes a laser chip 12, a heat sink 11 on which the laser chip 12 is attached, and an external mirror 13 spaced apart from the laser chip 12. The VECSEL 10 further includes a pump laser 15 aligned at an angle to provide a pump laser beam to the laser chip 12. Though not shown, the laser chip 12 includes a distributed Bragg reflector (DBR) and an active layer stacked sequentially on a substrate. As known to those of ordinary skill in the art, the active layer, for example, has a multi quantum well structure with a resonant periodic gain (RPG) structure and is excited by a pump laser beam to generate light at a predetermined wavelength. The heat sink 11 cools the laser chip 12 by dissipating heat generated by the laser chip 12. The pump laser 15 irradiates light (having a shorter wavelength than the light emitted from the laser chip 12) to the laser chip 12 through a lens array 16 in order to activate the active layer of the laser chip 12.
In this structure, when light emitted from the pump laser 15 at a relatively short wavelength is incident on the laser chip 12 through the lens array 16, the active layer of the laser chip 12 is activated to generate light at a specific wavelength. The light generated by the active layer is repeatedly reflected between the DBR layer of the laser chip 12 and the external mirror 13 through the active layer. Therefore, a resonance cavity of the VECSEL 10 is defined between the DBR layer of the laser chip 12 and a concave surface of the external mirror 13. Through this repeated reflection, the light is amplified in the laser chip 12. A portion of the amplified light is output to the outside as a laser beam through the external mirror 13, and the remainder of the light is reflected again to the laser chip 12 as pump light. A second harmonic generation (SHG) crystal 14 can be located between the laser chip 12 and the external mirror 13 to double the frequency of the light. If the SHG crystal 14 is used, for example, a laser beam having a visible wavelength can be output from infrared light emitted from the laser chip 12.
In the VECSEL 10 with this structure, however, the pump laser 15 that activates the laser chip 12 does not share a common axis with the other components, but must be offset from and at an angle to the axis. This increases the complexity and time of the manufacturing process, thereby rendering mass-production of the VECSEL difficult, and also limits the potential size reduction of the VECSEL. Further, the pump laser beam emitted from the pump laser 15 is incident on the laser chip 12 at about 45 degrees, thereby causing significant reflection loss and a drop in oscillation efficiency. In addition, since the pump laser beam has an elliptical shape on the laser chip 12 due to the incident angle, light generated by the laser chip 12 in response to the pump laser beam has an elliptical cross-section instead of a circular cross-section.
Further, since the wavelength converting efficiency of the SHG crystal 14 increases in proportion to incident light energy, it is preferable that the SHG crystal 14 is positioned close to the laser chip 12. However, this is not possible because the pump laser beam of the pump laser 15 is incident on the laser chip 12 from the front of the laser chip 12. Therefore, the efficiency of the SHG crystal 14 decreases.